What is an LED?
Where have LEDs been used in the lighting industry?
Have LEDs always been used in general illumination lighting?
Why have past attempts to create general illumination LEDs failed?
Why don’t LEDs function as efficiently in a traditional fixture housing?
What are the advantages to using LED lights?
What’s the difference between efficiency and efficacy?
How do you evaluate LED products?
How are LEDs able to outperform HID?
How does ambient temperature affect LED efficiency?
What is junction temperature?
Do I have to replace LED diodes?
Why is the life span of an LED measured as lumen depreciation?
How long is 50,000 hours?

What is an LED?
LED is the common abbreviation for a light-emitting diode. Each individual LED consists of a semiconductor diode that emits light when a voltage is applied to it. The electronics industry has used LED technology for several decades as indicator lights for various electronic devices. In more recent years, LED technology has progressed to the point where it is viable for general lighting applications.

Where have LEDs been used in the lighting industry?
LEDs are commonly used in aesthetic, effect, or specialty lighting applications, including architectural highlighting. Many traffic lights and exit signs, for example, now use LEDs.

Have LEDs always been used in general illumination lighting?
No. Early attempts to apply LEDs in general illumination lighting failed because they didn't meet the lumen-per-watt output or color requirements. Technology has advanced to the point where using LEDs for general illumination is now viable. Lighting industry experts are gaining a better understanding of how to capitalize on that technology.

Why have past attempts to create general illumination LEDs failed?
Conventional approaches to developing general illumination LEDs often involved retrofitting existing fixtures to house the new LED technology. Instead of investigating the benefits and challenges of LEDs, many early attempts simply used traditional lighting standards and housings. The problem was that LED technology breaks all traditional rules, and it quickly became apparent that old thinking couldn’t be applied to this new technology.

Why don’t LEDs function as efficiently in a traditional fixture housing?
An LED module may physically fit into an existing housing, but that housing doesn’t leverage the inherent qualities of the LEDs. Standard housings can’t handle the challenges of LED thermal management, which is vastly different than thermal management for traditional incandescent or fluorescent lighting. Also, the optical design used in most traditional fixtures doesn’t maximize the LED’s efficiency.

What are the advantages to using LED lights?
LEDs bring several advantages to the lighting industry, including high efficiency and durability, and, with superior life over other lamp sources, their required maintenance is greatly reduced. This translates into energy savings, maintenance savings and an overall reduction in cost of ownership over the product’s lifetime.

What’s the difference between efficiency and efficacy? Efficacy is a term normally used in cases where the input and output units differ. In lighting, we are concerned with the amount of light (in lumens) produced by a certain amount of electricity (in watts).

On the other hand, efficiency is a term that is typically dimensionless. For example, lighting fixture efficiency is the ratio of the total lumens exiting the fixture to the total lumens initially produced by the light source.

How do you evaluate LED products?
Lumen output is only part of the story and can be misleading. To fully evaluate an LED product one needs to review the overall system efficiency, optical control, thermal management of the LEDs, and know at what point in time the fixture will reach 30 percent lumen depreciation. Products with good optical efficiency and thermal management will be able to deliver more lumens, on average, than traditional HID products.

As the China and US Department of Energy concluded in its Solid-State Lighting (SSL) Commercial Product Testing Program:

“Until the field of SSL technologies and supporting knowledge matures, any claims regarding performance of SSL luminaires should be based on overall luminaire efficacy (i.e., from testing of the entire luminaire, including LEDs, drivers, heat sinks, optical lenses and housing), to avoid misleading buyers and causing long-term damage to the SSL market.”